Microcapsular opacifiers and method for their production

ABSTRACT

Air-containing microcapsular opacifiers, optionally containing pigment particles incorporated in the microcapsular structure, have substantially continuous, organic polymeric solid walls and a particle diameter of about 0.5 micron to about 10 microns, e.g., less than 1 or 2 microns. The opacifiers are produced by heating precursor microcapsules containing a liquid core material to a temperature sufficient to substantially drive off the core material from the precursor microcapsules and replace it with air.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of copending application Ser. No.262,894, filed June 14, 1972, now abandoned which in turn is a divisionof copending application Ser. No. 852,055, filed Aug. 21, 1969, now U.S.Pat. No. 4,007,141, issued Feb. 8, 1977 said application Ser. No.852,055 being a continuation-in-part of application Ser. No. 632,392,filed Apr. 20, 1967, now abandoned.

BACKGROUND OF THE INVENTION

Optical opacity, for example, "hiding" of a paint film, is a propertywhich is of great importance in the coating industry where such filmsare employed to decorate and protect the substrate on which they areapplied. In many such applications, such as those in the applianceindustry, white finishes of high hiding and having good protectivequalities are desired, and special effort has been expended in order toproduce suitable coatings of this type.

The optical opacity of films is achieved either by absorption of theincident light or by scattering of the incident light or a combinationof these two. Thus, black is opaque because it absorbs the lightincident on it and white is opaque because it back-scatters the incidentlight. Light is either absorbed or scattered before it can reach thesubstrate. The ideal white coating, therefore, is one which has zeroabsorption and maximum scattering.

Opaque films have conventionally been prepared by adding pigment, suchas titanium dioxide, to a film-forming material which would otherwise becolorless or transparent when cast in a film. The necessity for addingan opacifying agent obviously increases the cost of the resultant film.Moreover, the addition of such materials often causes the impairment ofthe physical properties of the resultant film.

More recently, processes have been described in the art for preparingopaque films which rely for opacity upon the presence of a large numberof cells or voids in the film. One such method involves depositing afilm from an emulsion, such as an oil-in-water or a water-in-oilemulsion. Another technique for obtaining such cellular films is bypreparing an aqueous dispersion of a film-forming polymer containing awater-soluble organic solvent solvent in an amount which is insufficientto dissolve the polymer. This aqueous dispersion is then cast as a filmand water is evaporated, thereby causing entrapment of minute dropletsof the organic solvent in the polymer. The film is then washed todissolve the entrapped minute droplets of solvent and the film is dried.

Opaque films having such microscopic voids display improved hiding powerover films utilizing pigments and, moreover, are less costly and possessimproved physical properties. But despite the over-all utility of filmsproduced according to this method, their use has been handicapped by thedifficulties inherent in the preparation of the film-formingcomposition. When an emulsion technique is employed, for example, caremust be taken in order to insure its stability, i.e., so that it willnot break before it is used to deposit a film. This frequently requiresthe use of emulsifying agents. However, emulsifying agents which arethen present in the film detract from the physical properties of thefilm, such as its water repellency, scrub resistance, etc.

More generally, the formation of microvoids according to conventionalpractice usually involves a spontaneous and non-controllable process.Consequently, wide variations in the size of the voids and theirdistribution in the film-forming material often result, with anaccompanying decrease in both opacity and film characteristics.

In brief, it has not been possible according to any known procedures toproduce an opaque, non-pigmented coating or film without employingrelatively elaborate film-forming methods, as with the above-describedtechniques, or without foregoing desirable film characteristics, as hasusually occurred with the use of pigments.

SUMMARY OF THE INVENTION

It has now been discovered that opaque films, having superior opacityand whiteness and other desirable properties, can be obtained fromcompositions prepared according to a very easily carried out processwhereby there is distributed throughout a liquid resinous film-formingbinder material particulate matter which contains one or more cellswithin each particle, the cells having an average size of from about0.01 micron to about 15 microns.

In addition to obviating the necessity for elaborate techniques requiredfor the preparation of present opaque, non-pigmented coatings, thecompositions of this invention have several additional advantages. Forinstance, the cell size can be more easily controlled so that, ifdesired, the composition can be prepared utilizing particles of uniformdiameter, thereby assuring uniform cell size in the film. Moreover, bothprior to and after the combining of the components, the system ischaracterized by an over-all ease of handling, requiring no stringent orelaborate controls. The particulate cellular material, which can beconveniently utilized in the form of a free-flowing powder, is simplyground into, blended, or otherwise conventionally combined with thefilm-forming material. The resulting mixture presents the same ease ofhandling usually associated only with fully pigmented compositions.

The compositions of this invention are formed into films and dried byconventional techniques. By "drying" is meant producing a relativelyhard, dry film; depending upon the film-forming material utilized, thismay require only evaporation of solvents, or coalescence or chemicalreaction resulting from oxidation or the application of heat or a curingagent may be required. Any such film-forming mechanism can be employedto produce films in accordance with the invention herein. When utilizedas coating compositions, the compositions herein can be applied tonumerous substrates, including, e.g., steel, aluminum, and other metals,as well as wood, plastic, paper, and the like. They may be coated ontosuch substrates by brushing, spraying, dipping, roller coating, knifecoating, and the like, and air-dried, air-cured, vacuum dried, or bakedat elevated temperatures.

Films prepared according to the simple process of this invention, havingthicknesses ranging up to about 20 mils, display excellent properties,such as extreme opacity or hiding power, and may be utilized ascoatings, for instance, as automotive finishes, appliance finishes,coatings for lighting fixture reflectors, and decorative coatings.Cross-linked coatings of this type are insoluble and infusible and areextremely tough and abrasion resistant. The coatings are especiallyuseful in lighting fixtures since they almost completely reflect lightrather than absorb it.

Microporous films of this invention, particularly those having anaverage cell size of less than 0.1 micron, are also useful in a varietyof applications as free films. For example, they can be utilized asvapor or liquid permeation membranes. One such use for such permeationmembranes is in desalinization processes.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows a cross-sectional view of an opaque layer of a cured resincomposition having distributed therein numerous discrete cellularparticles. As shown, the substrate 1 has thereon, as a coating, a cured,opaque and substantially non-pigmented film 2 prepared according to themethod of this invention. Distributed throughout the film-formingmaterial 3 are numerous single-celled, air-filled particles 4 having anoutside diameter of 0.5 microns and an average cell diameter of 0.3microns. Multicellular particles 5 having an average particle size of 10microns are also present, along with the single-celled particles, toscatter incident light and thereby provide an opaque film.

DESCRIPTION OF PREFERRED EMBODIMENTS

In accordance with the invention, opaque films are prepared fromresinous compositions which comprise liquid resinous film-forming bindermaterial and particulate cellular matter comprising discrete solidparticles which contain therein cells having an average size betweenabout 0.01 micron and about 15 microns, the particles being composed ofa substantially non-opaque material.

The film-forming materials which may be used in the practice of thisinvention include any such material, many of which are well known in theart. Such materials must contain at least one film forming polymer. Suchpolymers include thermoplastic and thermosetting, synthetic and naturalpolymers.

The invention is useful in opacification of films made from film-formingmaterials which are substantially non-light absorbing. Such materialsform dried films which are clear or translucent; light-absorbingcompositions which are black or dark in color are not ordinarily usableto provide films and coatings in accordance with the present invention.

Examples of film-forming materials useful in this invention includethose prepared from cellulose derivatives, e.g., ethyl cellulose,nitrocellulose, cellulose acetate, cellulose propionate and celluloseacetate butyrate; acrylic resins, e.g., homopolymers and copolymers witheach other or with other monomers of acrylic or methacrylic acid andtheir derivatives, such as methyl acrylate, methyl methacrylate, ethylacrylate, ethyl methacrylate, butyl acrylate, butyl methacrylate,acrylamide, acrylonitrile, etc; polyolefins, e.g., polyethylene andpolypropylene; polyamides, such as nylon, polycarbonates; polystyrene;copolymers of styrene and other vinyl monomers such as homopolymers andcopolymers of vinyl acetate, vinyl chloride and vinyl butyral;homopolymers and copolymers of dienes such as polybutadiene,butadiene-styrene copolymers, and butadiene-acrylonitrile copolymers.

Condensation polymers may also be used, such as alkyd resins, which areobtained by the condensation of a polyhydric alcohol and apolycarboxylic acid. Examples of polycarboxylic acids which may be usedto form the alkyd resin include phthalic acid, succinic acid, adipicacid, maleic acid, isophthalic acid, terephthalic acid, etc., which arereacted with polyhydric alcohols such as ethylene glycol, propyleneglycol, glycerine, sorbitol, pentaerythritol, and the like. Epoxy resinsmay also be used as the film-forming material. Epoxy resins include thecondensation products of bis-phenol and epichlorohydrin, epoxidizedoils, the glycidyl ethers or glycerol, epoxylated "novolac" resins, etc.Phenolic resins, such as those obtained by the reaction of phenol andformaldehyde, may also be used, as can aminoplast resins derived fromthe reaction of a compound containing a plurality of --NH₂ groups (e.g.,urea, melamine, guanamine or benzoguanamine) with an aldehyde or asubstance acting as an aldehyde (e.g., formaldehyde, benzaldehyde,paraformaldehyde). In preparing aminoplasts, the aldehyde or itsequivalent is usually dissolved in an alkanol, such as butyl alcohol,and at least a part of the N-methylol groups on the aminoplast areconverted into ##STR1## groups.

A preferred group of film-forming materials which may be used in thepractice of this invention are carboxylic acid amide interpolymers ofthe type disclosed in U.S. Pat. Nos. 3,037,963; 3,118,853; 2,870,116;and 2,870,117, the disclosures of which are incorporated herein byreference. These interpolymers are prepared by forming an interpolymerof an unsaturated carboxylic acid amide, such as acrylamide ormethacrylamide, with at least one other polymerizable ethylenicallyunsaturated monomer, and then reacting the interpolymer with analdehyde, such as formaldehyde, in the presence of an alcohol, such asbutanol.

Alternatively, such interpolymers can be produced by first reacting theunsaturated amide with an aldehyde and, if desired, an alcohol, to forman N-alkylol or an N-alkoxyalkyl-substituted amide. The N-substitutedamide then is interpolymerized with the other monomer or monomers,thereby producing interpolymers having the aforesaid recurrent groupswithout the need for further reaction. Such a method utilizingN-alkoxyalkyl-substituted amides is described in U.S. Pat. No.3,079,434.

Advantageous properties are often obtainable by employing mixtures ofthe above amide interpolymer resins with other resinous materials, suchas many of those mentioned herein. For example, nitrocellulose,polyethylene, alkyd resins, epoxy resins, aminoplast resins, and otherscan be utilized for this purpose.

Another preferred group of film-forming materials which may be used inthe practice of this invention consists of interpolymers ofhydroxyl-containing esters of unsaturated acids with at least one otherpolymerizable ethylenically unsaturated monomer. Such interpolymers areprepared, for example, by the free-radical initiated polymerization of amixture of monomers comprising at least 2 percent by weight of ahydroxyalkyl ester of an ethylenically unsaturated carboxylic acid,generally having up to about 12 carbon atoms in the alkyl group, and atleast one other ethylenically unsaturated monomer copolymerizabletherewith. In many cases, more than one hydroxyalkyl ester is includedin the interpolymer, and generally several monomers in addition to thehydroxyalkyl ester or esters are employed. Preferred hydroxyalkyl estersof ethylene glycol and 1,2-propylene glycol, i.e., hydroxyethyl acrylateand methacrylate, and hydroxypropyl acrylate and methacrylate, but theremay also be employed similar esters of other unsaturated acids, forexample, ethacrylic acid, crotonic acid, and similar acids having up toabout 6 carbon atoms, as well as esters containing other hydroxyalkylradicals, such as hydroxybutyl esters and hydroxylauryl esters, and themono- or diesters of unsaturated dicarboxylic acids, such as maleicacid, fumaric acid, and itaconic acid, in which at least one of theesterifying groups is hydroxyalkyl, such as hydroxyethyl hydrogenmaleate and bis(hydroxypropyl)fumarate.

The monomer or monomers with which the hydroxyalkyl ester isinterpolymerized can be any ethylenic compound copolymerizable with theester. These include monoolefinic and diolefinic hydrocarbons,halogenated monoolefinic and diolefinic hydrocarbons, unsaturated estersof organic and inorganic acids, esters of unsaturated acids, nitriles,unsaturated acids, and the like. Examples of such monomers includestyrene, butadiene-1,3,2-chlorobutene, alpha-methyl styrene,2-chlorobutadiene-1,3, vinyl butyrate, vinyl acetate, dimethyl maleate,divinyl benzene, diallyl itaconate, and the like. Preferred comonomersare the alkyl esters of ethylenically unsaturated carboxylic acids,vinyl aromatic hydrocarbons, ethylenically unsatuated nitriles, andethylenically unsaturated carboxylic acids. The specific comonomers mostoften employed are methyl methacrylate, ethyl acrylate, styrene, vinyltoluene, acrylonitrile, methacrylonitrile, methacrylic acid, acrylicacid, 2-ethylhexyl acrylate, butyl acrylate, butyl methacrylate, andlauryl methacrylate.

Films are usually produced from the above interpolymers of hydroxyalkylesters by cross-linking these interpolymers with another materialcontaining functional groups reactive with the hydroxyl group of theinterpolymer, such as, for example, polyisocyanates and aminoplastresins.

Still other film-forming materials which may be used in the practice ofthis invention include, for example, naturally occurring materials suchas casein, shellac and gelatin.

The particles which constitute the particulate matter employed in theinvention are discrete, solid entities which contain therein one or morecells. The walls of each particle are composed of a material which issubstantially non-opaque; i.e., the particle walls are transparent ortranslucent to incident light rays in order to enable the particle toact as a light scattering agent.

The overall size of the particles is relatively noncritical, butgenerally speaking, they should not have a size (measured by the maximumdimension) of greater than about 100 microns. The size of the particlesdepends in large part upon the type of particles, the number of cells ineach, etc. For instance, monocellular particles ordinarily have anaverage size of from about 1 to about 10 microns, while multicellularparticles have an average size of 3 to 30 microns or larger.

Substantially all of the cells or voids have diameters ranging from aslow as about 0.01 micron up to about 30 microns, with the average cellsize being up to about 15 microns. Preferably, the cells are less thanabout 5 microns, and an especially preferred diameter range is fromabout 0.01 micron to about 0.8 micron. Films prepared according to theprocess of this invention but having cell sizes outside of theabove-disclosed diameter range do not function as efficient lightscatterers and do not provide the excellent film characteristics asexhibited by the coatings herein disclosed.

The particles of this invention can be prepared having either continuousor non-continuous cell voids; e.g., the cells can be open (connected) orclosed (unconnected). Since the opacity of the film is dependent uponthe presence of a substantial number of cells, or voids, within the filmitself, it is essential that the film-forming material be prevented fromentering into the cells so as to completely fill them. Thus, whenpermeable or open-cell structured particles are utilized, a suitablenon-penetrating film-forming material must be selected so as to allowthe formation therein of a sufficient number of voids of the sizedescribed.

The cells or voids in the particulate matter, as those terms are usedherein, refer to spaces within the particulate matter not containingsolid or liquid matter after drying of the film. The cells (or voids)are most commonly filled with air, although any suitable gas or mixtureof gases may be employed in place thereof. Thus, when reference is madeto gas or air-filled cells, it should be clear that other gases, such ashelium, nitrogen, carbon dioxide, dichlorodifluoromethane, and the like,are also contemplated as being within the scope of the invention.Similarly, the cells can also be prepared in a vacuum state, and thesetoo are included. In certain cases gas-filled cells are provided in theparticles as produced and these are combined with the binder material toform the composition used to make opaque films. In other cases, however,there are utilized particles with cells containing a volatile liquid ora sublimable solid (formed by encapsulation techniques, for instance)and these are combined with the binder. Upon drying, the volatile liquidor sublimable solid vaporizes, leaving gas-filled voids in the driedfilm.

The particulate matter of this invention can be prepared from anysubstantially non-opaque material which can be formed into cellularparticles of the size and structure as described above and which retainthis form during the drying of the film-forming material. Suchparticle-forming material encompasses virtually any natural or syntheticresinous material including, for example, substantially all of thematerials described above.

That is, the particle-forming material can be prepared from materialssuch as cellulose derivatives, for example, cellulose acetate, celluloseacetate-butyrate, and cellulose acetate-propionate; acrylic resins, suchas homopolymers and copolymers with each other, or with other monomerssuch as methylacrylate, methyl methacrylate, ethyl acrylate, ethylmethacrylate, butyl acrylate, butyl methacrylate, hydroxyethyl acrylate,hydroxypropyl methacrylate, acrylic acid, methacrylic acid, acrylamideand acrylonitrile; polyolefins, such as polyethylene and polypropylene;polyamide resins, such as those of the Nylon "66" type; polycarbonates,polystyrenes; copolymers of styrene and other vinyl monomers, forinstance, acrylonitrile; vinyl polymers, such as homopolymers andcopolymers of vinyl acetate, vinyl alcohol, vinyl chloride, and vinylbutyral; homopolymers and copolymers of dienes, such as polybutadiene,butadiene-styrene copolymers and butadiene-acrylonitrile copolymers;alkyd resins; polysiloxane resins; phenol-formaldehyde resins,urea-formaldehyde resins; and melamine-formaldehyde resins. All of theseresins are film-forming and can be hardened or cured to a substantiallynon-opaque material and, therefore, are capable of forming discretecellular particles capable of scattering light.

Natural resinous materials are also included within the scope of thisinvention for the preparation of the particulate matter, includingsoybean protein, zein protein, algenates, and cellulose in solution, andcellulose xanthate or cuprammonium cellulose. Inorganic film-formingsubstrates such as sodium silicate, polyborates and polyphosphates, arealso contemplated as being within the scope of the above term.

The particulate matter of this invention can be prepared by variousmethods, including microencapsulation techniques as well as processeswhich require the use of blowing agents. Encapsulation methods are oftenespecially suitable, and can be used to provide particles with eithergas-filled cells or cells containing a volatile liquid or sublimablesolid. A typical process is disclosed, for example, in U.S. Pat. No.3,173,878, the disclosure of which is incorporated herein by reference.Polymeric microcapsules are produced by first creating a dispersion ofan aqueous or other polar solvent solution in a solution of ahydrophobic encapsulating polymer in a non-aqueous non-polar solventliquid. The polar or aqueous solution is present in the form ofcolloidal droplets which constitute the disperse phase, with thesolution of the polymer constituting the continuous phase. Upon adding asecond non-aqueous non-polar liquid, miscible with the polymer solutionbut in which the polymer is itself insoluble, the polymer is caused toprecipitate around the droplets of the aqueous or polar solvent solutionto form the polymeric microcapsules. These particles are suspended astiny polymeric bodies in an equilibrium mixture of the added liquid andthe continuous phase liquid.

As is common to most conventional encapsulation methods, such particlesare then separated and gradually insolubilized and hardened by washingwith successive liquid mixtures, each comprising intermixed solvent andnon-solvent for the polymer, to form, upon drying, a finished productconsisting of liquid filled microcapsules.

The liquid is usually prevented from being released through the pores ofthe polymeric wall material of the capsule, according to such methods,by means of various techniques, such as, for example, rapid performingof the gelation step. In the present invention, however, it is essentialthat the particles, when prepared according to such a method, releasethe liquid prior to or during the curing or hardening of thefilm-forming material in which the particles have been dispersed.Otherwise, a cellular film will not result and the cured film will benon-opaque. Thus, for the purposes of the present invention, thecapsules are prepared to allow the timely release of the encapsulatedfluid and the production of gas-filled microcapsules.

Particulate matter of this invention can be prepared containing aliquid, as described above, in order to give the particles more density,so that they can be more readily dispersed in the film-forming material.For example, capsules containing volatile liquids such as toluene orxylene are suitable for such use. Upon curing or drying of thefilm-forming material, in which the particles are dispersed, the liquidis released from the capsules to provide a film having the desiredproperties of opacity.

Similarly, microencapsulation techniques may be utilized which requireor are most amenable to the encapsulation of solid particles. Here it isonly necessary to use a solid which can sublime, i.e., be transformeddirectly to the vapor state without passing through the liquid phase,within a suitable temperature-pressure range. Such encapsulated solidparticles can then be utilized as the particulate matter of thisinvention by dispersing them in the film-forming composition, applyingsuch composition to a substrate, and curing or drying the composition toalso cause the solid material within the capsules to sublime. Theresultant cured or hardened film is thereby characterized by a cellularstructure and is, accordingly, opaque.

A specific process for the preparation of unitary and discretesingle-celled particles having a thin, strong skin, being substantiallyspherical in shape and which are substantially free from holes isdisclosed in U.S. Pat. No. 2,797,201, the disclosure of which isincorporated herein by reference. In such technique, a solutioncomprising a volatile solvent having dissolved therein film-formingmaterial and a latent gas is subdivided into droplets and the dropletsare then subjected to a drying temperature at which the solvent isvolatized and a hole-free tough surface skin is formed on the particles,and at which the latent gas is converted into a gas. In this way, gas isliberated within the particle coincident with its formation and istrapped beneath the surface skin of the particle, and either forms ahollow space there within or finds its way into a hollow space otherwiseformed therein and through its presence there tends to prevent collapseof the particle walls under pressure of the atmosphere. Theconcentration of the solution of film-forming material, according tothis technique, is not ordinarily critical but for the purposes of thepresent invention it is usually required or preferred to employ lowconcentrations since the smallest particles are formed from dilutesolutions.

A suitable apparatus for the preparation of the above-describedparticles is disclosed in U.S. Pat. No. 3,230,064, the disclosure ofwhich is incorporated herein by reference. Such apparatus introduces thefeed material, that is, the solution comprising the volatile solventhaving dissolved therein film-forming material and latent gas, near thebottom of a conventional furnace or spray drying device into anascending column of hot furnace gases. The feed material is admitted insubdivided form and is entrained in an upward moving, hot, gaseousstream. By such method, it is possible to prepare the particulate matterof this invention within the required narrow particle size range, thatis, of a size less than 15 microns average diameter.

Another method widely utilized in the art for producing cellularparticulate matter comprises the heat curing, while falling freelythrough space, or being conveyed by gas stream, of a granule or liquiddroplet of a material capable of condensing to a thermosetting resinwhile concomitantly liberating a gas. Typical materials for preparingsuch particles, for example, are hexamethylenetetramine and phenol. Amore detailed description of this technique is described in U.S. Pat.No. 2,929,106, the disclosure of which is incorporated herein byreference.

A method for preparing multicelled particles for employment as theparticulate matter of this invention is disclosed in the Society ofPlastics Engineers Journal, Volume 17, No. 3, March 1961, pages 249-251,the disclosure of which is incorporated herein by reference. Here, anyresinous composition that can be foamed is introduced under pressureinto a stream of hot gases in a gas turbine generator to form smokesconsisting of spherical, foamed plastic particles having numerous cellstherein. Powders of these particles can be obtained using this smokemaking process by means of placing particle collection equipment in thepath of the smoke; the powder thus constituting particulate matter forutilization as herein disclosed. Alternatively, the smoke can bedirectly absorbed into a liquid, a paint vehicle or other film-formingmaterial or the like, to completely avoid the necessity of preparing apowdered form of the smoke particles.

Numerous processes are similarly available for the production ofinorganic cellular particles, such as the technique disclosed in U.S.Pat. No. 2,978,340, the disclosure of which is incorporated herein byreference. Here there is described a method for preparing discrete,hollow glass spheres from a synthetic mixture of a siliceous material, awater desensitizing agent, and a compound which liberates a gas at afusion temperature for this mixture. The process consists of subjectingparticles of this mixture to an elevated temperature for a timenecessary to fuse the particles and cause expansion of the particlesinto spheres. Additional information regarding the preparation of glassparticles may be found in U.S. Pat. Nos. 3,030,215 and 3,129,086, thedisclosures of which are also incorporated herein by reference.

As described above, a most significant advantage of the process of thisinvention is the comparative simplicity with which the film-formingcomposition may be formed. For example, the particulate matter asprepared above can be simply mixed into a paint vehicle or other form offilm-forming material and the combination applied to a substrate andhardened or cured to form a resultant opaque film. No unusual orelaborate steps are required to obtain a satisfactory film-formingcomposition, or to make a free film or coating therefrom.

The amount of particulate matter employed in the composition varies,depending, for instance, upon the opacity and particular application.Preferably the particulate matter is present in amounts sufficient toprovide a cellular volume which constitutes from about 30 percent toabout 90 percent of the total volume of the film-forming material andparticulate matter, an especially preferred range being a cellularvolume of from about 40 percent to about 70 percent of this totalvolume. By "cellular volume" is meant the total volume of the voids orcells having the specified cell size.

Ordinarily the films produced using the cellular particles as the onlyopacifying agent are opaque and white. Color forming material such asdye or pigment can be included in the film-forming composition, ifdesired, either in the wall material of the particles or in thefilm-forming material, to produce colored films. Only small amounts ofdyes or pigments are employed for this purpose. It is often advantageousto employ particulate matter, as disclosed above, in combination withconventional pigments such as, e.g., titanium dioxide, carbon black,talc, barytes and the like, as well as conventional color pigments, suchas e.g., cadmium yellow, cadmium red, phthalocyanine blue, chromicyellow, toluidine red, and the like. By blending these dissimilaropacifying substances, it is possible to obtain film characteristics,e.g., whiteness, utilizing much lower amounts of pigment. However, whenpigment is thus utilized, the amount is less than is normally requiredto opacify the film-forming material used, i.e. less than the amount ofpigment required to provide an equivalent degree of opacification ofsuch a film in the absence of the cellular particles. This is especiallyvaluable where the specific pigment employed is of high cost or hasother undesirable features when employed in higher amounts.

Thus, the important aspect of the invention is the use of the cellularparticles described in place of all or part of the pigment which wouldotherwise be required to opacify a dried free or adherent film to theextent desired. Opacification of the dried film is obtained in thismanner despite the fact that the cellular particles are themselvesnon-opaque, and even though the composition containing the binder andthe particles may be non-opaque.

The examples which follow serve to further illustrate the invention, butshould not be construed as imposing limitations thereon. All parts andpercentages are by weight and are based upon non-volatile solidscontent, unless otherwise indicated.

EXAMPLE 1

Particulate matter consisting of discrete polyamide particles as a 20percent emulsion in water, the particles having cell diameters of about1 micron, was utilized according to this invention in preparing opaque,non-pigmented films. The particles were made of nylon (polyamide fromhexamethylenediamine and adipic acid) and contained xylene within theparticle cells to enable them to be more readily dispersed in thefilm-forming material.

A polyvinyl acetate latex composition, consisting of 75 percent of vinylacetate and 25 percent of dibutyl maleate, served as the film-formingmaterial. The latex composition had a pH of 4.0 to 5.0, a viscosity(Brookfield) of 500 to 1000 centipoises at 77° F., and contained 55percent solids in water.

The above two components, in amounts of 116 grams of particulate matteremulsion and 12 grams of latex, were added to a 250 milliliter flaskequipped with a stirrer. The materials were blended well for about 10minutes, following which a 6 mil film of the mixture was drawn onto a 9"by 12" glass plate which had previously been cleaned with toluene andacetone. The coated glass plate was air dried at room temperature forabout 2 hours. The hardened film obtained contained cells from theevaporation of the xylene; it was white and displayed excellent hiding.

EXAMPLE 2

An opaque, non-pigmented film was prepared accordint to the procedure ofExample 1, utilizing substantially the same materials, except that thepolyamide particles had an average cell diameter of about 25 microns.The particulate matter emulsion, in an amount of 116 grams, was blendedfor about 30 minutes with 12 grams of the polyvinyl acetate latexcomposition and the resulting product was then drawn in a 3 mil wetthickness onto a 9" by 12" glass plate previously cleaned with tolueneand acetone. The coated plate was air dried at room temperature forabout 2 hours. The tack-free, microporous film obtained was uniformlyopaque.

EXAMPLE 3

Particulate matter suitable for use in the invention was prepared byheating 500 grams of water and 5 grams of acacia gum to 80° to 85° C.with stirring. A solution of 20 grams of epoxy resin ("ERL-2772", areaction product of Bisphenol A and epichlorohydrin having an epoxideequivalent of 175 to 190) and 2 grams of ethylene diamine was heated to75° C. and slowly added to the above prepared aqueous phase over a 20minute interval. The temperature was controlled at 85° C. duringaddition. Heating at 80° to 85° C. was continued for 2 hours and themixture was then allowed to cool to room temperature after which atwo-layered product was obtained, the top layer consisting of theparticles.

The particles were then washed with water and blended with a polyvinylacetate latex substantially the same as that utilized in Example 1. Thethick, homogeneous product of the blend was then drawn onto a glassplate (9"×12") in a 6 mil wet film thickness and air dried at roomtemperature over about a two hour period. The resulting microporous filmwas uniformly opaque.

Numerous other particulate matter compositions and techniques, as wellas a wide variety of other film-forming materials, can be utilizedaccording to the teachings of this invention. For example, particulatematter in the form of hollow, hole-free particles can be prepared from apolyvinyl alcohol solution according to the procedure disclosed in U.S.Pat. No. 2,797,201. The particles formed thereby are substantiallyglobular in shape, and can be dispersed, as above, in an acrylic resincomprising a copolymer of methyl methacrylate, lauryl methacrylate andmethacrylic acid. Films cast from such mixture are completely white upondrying.

Similarly, and as noted above, multicellular particles prepared fromphenol-formaldehyde resins can be prepared and utilized in the form of apowder and then combined with any of a number of various film-formingmaterials, such as an epoxidized polyester resin in a typical solventsuch as isopropyl acetate. Additionally, any of the single-celled ormulticelled particles described can be utilized either in combinationwith or individually in any of the film-forming materials orcombinations thereof.

In accordance with the above description of the invention, the resinouscomposition herein disclosed can be readily used to form either as afree film or an adherent coating on any desired substrate, such as,e.g., steel, aluminum, wood, glass, plastic, and the like. This ispossible since the particulate matter utilized herein is completelyindependent of the film-forming material. That is, the discrete, hollowparticles can be blended or mixed into any desired substantiallynon-opaque coating composition to produce an opaque film.

According to the provisions of the patent statutes, there are describedabove the invention and what are now considered to be its bestembodiments. However, within the scope of the appended claims, it is tobe understood that the invention can be practiced otherwise than asspecifically described.

We claim:
 1. A method for the production of air-containing microcapsularopacifiers, which method comprises providing discrete, essentiallyspherical precursor microcapsules having substantially continuousorganic, polymeric solid walls, said microcapsules having an averageparticle diameter of below about one micron and containing a liquid corematerial, and heating said microcapsules to a temperature sufficient tosubstantially completely drive off said core material from saidmicrocapsules and replace said core material with air.
 2. A methodaccording to claim 1 characterized in that said liquid core material isa water-immiscible material.
 3. The method according to claim 2characterized in that the water-immiscible oily material is toluene. 4.A method according to claim 1 characterized in that the microcapsuleshave an average particle diameter of between about 0.10 and about 1.0micron.
 5. A method according to claim 1 characterized in that themicrocapsules have an average particle diameter of between about 0.25and about 0.8 micron.
 6. A method for the production of air-containingmicrocapsular opacifiers, which method comprises providing discrete,essentially spherical precursor microcapsules having substantiallycontinuous organic, polymeric, solid walls, said microcapsules having anaverage particle diameter of 0.5 micron, and containing a liquid corematerial, and heating said microcapsules to a temperature sufficient tosubstantially completely drive off said core material from saidmicrocapsules and replace said core material with air.
 7. A method forthe production of air-containing microcapsular opacifiers whichcomprises providing discrete, essentially spherical precursormicrocapsules having substantially continuous organic, polymeric, solidwalls, said microcapsules having an outside diameter of 0.5 micron, andcontaining a liquid core material, and heating said microcapsules to atemperature sufficient to substantially completely drive off said corematerial from said microcapsules and replace said core material withair.
 8. Microcapsular opacifiers comprising discrete, substantiallyspherical, air-containing microcapsules having substantially continuousorganic, polymeric solid walls, said microcapsules having an averageparticle diameter of below about 1 micron.
 9. Microcapsular opacifiersaccording to claim 8 characterized in that said opacifiers have anaverage particle diameter of between about 0.10 and 1.0 micron. 10.Micrcapsular opacifiers according to claim 8 characterized as having anaverage particle diameter of between about 0.25 and about 0.8 micron.11. Microcapsular opacifiers comprising discrete, substantiallyspherical, air-containing microcapsules having substantially continuous,organic, polymeric solid walls, said microcapsules having an averageparticle diameter of 0.5 micron.
 12. Microcapsular opacifiers comprisingdiscrete, substantially spherical, air-containing microcapsules havingsubstantially continuous, organic, polymeric, solid walls, saidmicrocapsules having an outside diameter of 0.5 micron.
 13. A method forthe production of microcapsular opacifiers which comprises providingdiscrete, essentially spherical precursor microcapsules havingsubstantially continuous organic, polymeric, solid walls, saidmicrocapsules having an average particle diameter of below about 2microns and containing a liquid core material, said microcapsulesadditionally having pigment particles incorporated in the microcapsularstructure, and heating said microcapsules to a temperature sufficient tosubstantially completely drive off said core material from saidmicrocapsules and replace said liquid core material with air.
 14. Themethod of claim 13 wherein said microcapsules have an average particlediameter of between about 0.10 and about 1.0 micron.
 15. The method ofclaim 13 wherein said microcapsules have an average particle diameter ofbetween about 0.25 and about 0.8 micron.
 16. The method of claim 13wherein said pigment is TiO₂.
 17. The method of claim 13 wherein saidliquid core material is a water-immiscible material.
 18. A method forthe production of microcapsular opacifiers which comprises providingdiscrete, essentially spherical precursor microcapsules havingsubstantially continuous, organic, polymeric, solid walls, saidmicrocapsules having an average particle diameter of 0.5 micron, andcontaining a liquid core material, said microcapsules additionallyhaving pigment particles incorporated in the microcapsular structure,and heating said microcapsules to a temperature sufficient tosubstantially completely drive off said core material from saidmicrocapsules and replace said liquid core material with air.
 19. Amethod for the production of microcapsular opacifiers which comprisesproviding discrete, essentially spherical precursor microcapsules havingsubstantially continuous organic, polymeric, solid walls, saidmicrocapsules having an outside diameter of 0.5 micron and containing aliquid core material, said microcapsules additionally having pigment insaid walls, and heating said microcapsules to a temperature sufficientto substantially completely drive off said core material from saidmicrocapsules and replace said liquid core material with air. 20.Microcapsular opacifiers consisting essentially of discrete,substantially spherical, air-containing microcapsules havingsubstantially continuous, organic, polymeric, solid walls and an averageparticle diameter of below about 2 microns, said microcapsulesadditionally having pigment particles incorporated in the microcapsularstructure.
 21. Microcapsular opacifiers as defined in claim 20 having anaverage particle diameter of between about 0.10 and 1.0 micron. 22.Microcapsular opacifiers as defined in claim 21 having an averageparticle diameter of between about 0.25 and about 0.8 micron. 23.Microcapsular opacifiers as defined in claim 20 wherein said pigment isan inorganic opacifying pigment.
 24. Microcapsular opacifiers as definedin claim 20 wherein said pigment is TiO₂.
 25. Microcapsular opacifiersas defined in claim 20 wherein said walls comprise a formaldehydecondensation product.
 26. Microcapsular opacifiers as defined in claim20 wherein said pigment is BaSO₄.
 27. Microcapsular opacifiersconsisting essentially of discrete, substantially spherical,air-containing microcapsules having substantially continuous, organic,polymeric, solid walls and an average particle diameter of below about 2microns, said microcapsules additionally having pigment particlescompletely encapsulated within the walls of the microcapsules. 28.Microcapsular opacifiers as defined in claim 27 having an averageparticle diameter of between about 0.10 and 1.0 micron. 29.Microcapsular opacifiers as defined in claim 28 having an averageparticle diameter of between about 0.25 and about 0.8 micron. 30.Microcapsular opacifiers as defined in claim 28 wherein said pigment isTiO₂.
 31. Microcapsular opacifiers consisting essentially of discrete,substantially spherical, air-containing microcapsules havingsubstantially continuous, organic, polymeric solid walls and an averageparticle diameter of about 1 micron, said microcapsules additionallyhaving pigment particles incorporated in the microcapsular structure.32. Microcapsular opacifiers consisting essentially of discrete,substantially spherical, air-containing microcapsules havingsubstantially continuous, organic, polymeric solid walls and a particlediameter of from about 0.5 micron to about 10 microns, saidmicrocapsules additionally having pigment particles incorporated in themicrocapsular structure.
 33. Microcapsular opacifiers consistingessentially of discrete, substantially spherical, air-containingmicrocapsules having substantially continuous, organic, polymeric solidwalls and a particle diameter of from 0.5 micron to about 5.0 microns,said microcapsules additionally having pigment particles incorporatedinto the microcapsular structure.
 34. Microcapsular opacifiersconsisting essentially of discrete, substantially spherical,air-containing microcapsules having substantially continuous, organic,polymeric solid walls, and an average particle diameter of 0.5 micron,said microcapsules additionally having pigment particles incorporated inthe microcapsular structure.
 35. Microcapsular opacifiers containingessentially discrete, substantially spherical, air-containingmicrocapsules having substantially continuous, organic, polymeric,solids walls and an outside diameter of 0.5 micron, said microcapsulesadditionally having pigment in said walls.